N'Guessan, signed on loan from Championship rivals Leicester, slotted home from 12 yards after strike partner John Marquis headed the ball on.
He came close to doubling his tally with 20 minutes remaining, his close-range header from a Jordan Stewart cross hitting a post.
N'Guessan could have scored inside 120 seconds but his effort was comfortably saved by Argyle goalkeeper Jake Cole, who also denied Chris Hackett from long range shortly afterwards.
Cole had to be alert again as half-time approached, punching away a Darren Purse header from Hackett's free-kick.
N'Guessan was presented with another opportunity early in the second half but could not convert Jack Smith's centre from six yards.
After he had been thwarted by the post, N'Guessan saw another shot saved by Cole.
Plymouth, meanwhile, struggled to carve out too many opportunities but nearly forced extra-time at the death when teenage substitute Matt Lecointe was denied by a point-blank save from Lions goalkeeper Steve Mildenhall.